[R] chisq.test

[Peter Ehlers]

On 2012-06-26 23:02, John wrote:
> On Wed, 27 Jun 2012 16:58:29 +1200
> Rolf Turner<rolf.turner at xtra.co.nz>  wrote:
>
>> On 27/06/12 08:54, arun wrote:
>>>
>>> Hi,
>>>
>>> The error is due to less than 5 observations in some cells.
>>
>>       NO, NO, NO!!!!  It's not the observations that matter, it is
>>       the ***EXPECTED COUNTS***.  These must all be at least
>>       5 in order for the null distribution of the test statistic to be
>>       adequately approximated by a chi-squared distribution.
>>
>>           cheers,
>>
>>               Rolf Turner
>
> Pretty sure the point was that in a situation where the expected counts
> are too low for a reliable chi-square, that an alternate test such as
> the nonparametric Fisher's Exact Test may be the way to go, especially
> if there isn't nay more data to get. That way you don't have to worry
> about expected counts.
>
> JDougherty

That may well be; nevertheless, the post included the statement
Rolf quotes: "... less than 5 _observations_ in some cells" (my
emphasis). And Rolf's point is quite correct - it's the
_expected_ counts that the approximation cares about.

Peter Ehlers